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What is the difference between turn rate and yaw rate in aircraft?
Attitude heading computers generate both a turn rate and a yaw rate. How are these different from each other? Is Euler yaw rate the same as turn rate?

Rate of turn is easy to define: it is the rate at which the direction of aircraft motion changes with respect to the ground (rate of change of track). If the aircraft is not slipping and there is no change in wind, then it is equivalent to the rate of change of heading.
Yaw rate, on the other hand, is slightly messier. Technically, yaw rate of the rate of change of the z-axis Euler angle. As an industry standard, the Euler angle in the aviation industry is defined by the z-y-x rotation, which means that the yaw occurs first, followed by pitch, followed by bank. Therefore, yaw rate is technically exactly equal to the rate of change of heading.
In many applications and loose technical speaks, however, yaw rate may also refer to the third component of the angular velocity. Angular velocity is the instantaneous rate of rotation about its axis of rotation. If the aircraft is rotating about the vertical axis only, then the third component of the angular velocity is, again, exactly equal to the rate of change of heading.
In a coordinated turn, however, the aircraft is pitched and banked, and therefore it is not rotating purely about the vertical axis. In this definition, then, yaw rate is not equal to turn rate.
As an aside, it's mathematically incorrect to call any component of the angular velocity a "rate", since angular velocity is not a derivative of any quantity, even though angular velocity can be transformed into Euler rates if you know the current Euler attitudes.

Turn rate is the rate at which an aircraft's direction of motion changes.
Yaw rate is:

If the aircraft is wings-level: the rate at which an aircraft's heading (the direction in which the aircraft's longitudinal axis points) changes.
If the aircraft is not wings-level: the rate at which the aircraft's heading would be changing if the aircraft were wings-level.  (To visualise this, roll your head until the aircraft appears to be wings-level, and then measure the rate at which its apparent heading1 changes.)

Ideally, the aircraft's turn rate and yaw rate are equal, so that the aircraft's heading stays aligned with its direction of motion through the air (this is good because the aircraft's profile area - the cross-sectional area that it presents to the oncoming air - is smallest when its direction of motion is aligned with its longitudinal axis, and the amount of pressure drag2 experienced by an aircraft is roughly proportional to its profile area; thus, aligning the aircraft's longitudinal axis and direction of motion minimises drag).  If the turn and yaw rates are equal and nonzero, this is called a coordinated turn.
It is also possible for the aircraft's turn rate to be different from its yaw rate:

For instance, if you step hard on the right rudder pedal while holding the yoke neutral, the aircraft will immediately yaw (change its heading) hard to the right, but will not start to turn (change its direction of motion) significantly until a large yaw angle has already developed; for the first few seconds, the aircraft has a large yaw rate but a negligible turn rate.
On the flip side of things, if you turn the yoke hard to the right while applying just enough left rudder to hold your heading (initially) constant, the aircraft will roll to the right and start slipping sideways through the air; this causes its direction of motion to change (becoming more rightward), introducing a nonzero turn rate, but, thanks to your left rudder input, the aircraft's heading remains constant, and the aircraft's yaw rate remains zero (until the aircraft's sideslip angle becomes large enough that you run out of rudder authority and the large right-yawing moment imposed by the vertical stabiliser forces the aircraft to yaw to the right).

1: The heading it would have if we redefined "up" and "down" to be aligned with the vertical axis of your head.
2: The drag that results from the aircraft having to push air out of its way as it moves through said air.  Also known as profile drag.

I have never heard yaw described as anything other than rotation about the vertical axis of the aircraft.  By your definition if you performed a coordinated immelmann or split S the aircraft would have yawed.  Do you agree with that?

"Yaw rate is the rate at which an aircraft's heading [...] changes." – Are you sure about that? To my knowledge, the words "roll," "pitch," and "yaw" always refer to intrinsic rotations, so that, for example, if I'm flying at a bank angle of 90° and I stomp on the rudder pedal, I yaw, even though my heading doesn't change.

@Terran, there is Euler yaw and body yaw, and they are different, hence the common confusion. For pilots, body yaw = yaw (as you say) and Euler yaw = heading (ignoring the question of zero direction; typically it's true heading).

@Zeus Oh right, sometimes I forget that "yaw" sometimes refers to an Euler angle.

This is a comparison of two frames of reference: turn rate is earth referenced on a horizon based yaw plane, whereas aircraft yaw rate is based on the aircraft yaw plane.
In a skidding turn with no bank, they would be equal.  In a 90 degree "bank and yank" or "knife-edge", the rate of turn is 100%  in the pitch plane, with 0 yaw.
A coordinated turn is a combination of pitch and yaw, producing a change in heading at a given rate.

To put it simply, turn rate is a change in aircraft heading, (over time) while yaw is rotation about the vertical axis of the airplane.
So, if the aircraft yaws the aircraft heading will also change.  However, the reverse is not true:  In a coordinated turn at a constant angle of bank the aircraft heading will be changing, but it will not be rotating about the vertical axis (unless you introduce a slip or skid) therefore the yaw rate should be zero.

That's incorrect. Yaw rate is steady state non zero during coordinated turn.

And an aircraft does rotate about its vertical axis during a coordinated turn - otherwise, its longitudinal axis would still lie in the plane defined by where its longitudinal and vertical axes were at the start of the turn.

See my comment to your answer below.

I agree that the longitudinal axis of the aircraft arcs, and changes position relative to the earth, my point is that the aircraft doesn't actually pivot about the vertical axis unless the ball is displaced.

I don't think either answer is correct. Yaw rate is not the rate of change of heading, nor is it true that the yaw rate is zero in a coordinated turn. During a coordinated turn to the left (for example), the plane undergoes a rotation which is a mixture of a pitch up and a yaw to the left. Despite pitching up and yawing left, the pitch and the angle of sideslip do not change.

I think the points being made are good ones, and I obviously concede that the aircraft "rotates" in a coordinated turn, but rather than rotating about the vertical axis of the aircraft, (yaw) it is rotating about a focal point at the center of the radius of turn. The axes are totally different, and the latter does not meet the traditional definition of yaw.

It doesn't matter where the center of rotation is. 1deg of rotation about my center of body is equal to 1deg of rotation about 1km away (even though the tangential distance moved is vastly different).

I agree it doesn't matter when you consider the heading change, but it definitely matters if you consider the textbook definition of yaw. A rotational axis a half mile away is not the same as the vertical axis running through the CG.  It is the difference between arcing and twisting.  They are distinctly different movements.

@MichaelHall But that reasoning seems to imply that an aircraft in flight can never yaw, since no matter what I do with the rudder pedals or whatever, the aircraft will rotate about an axis outside of itself. (Unless the aircraft is flying so slowly, and rotating so quickly, that some point on the aircraft remains motionless as the aircraft moves.)

@TerranSwett, not at all, if you move the the rudder it will yaw, just as if you move the elevator the aircraft will pitch and if you move the ailerons it will roll.   These forces act on axes that pivot about the center of gravity.  There is a good illustration and some explanation here:  http://ffden-2.phys.uaf.edu/webproj/211_fall_2016/Benson_Hoover/Slide6.htm

Admittedly I am unfamiliar with "Euler yaw", as a pilot I was taught the fundamentals of what Zeus refers to as "body yaw".  To clarify your point above, in a true body yaw in flight the axis isn't outside the aircraft, the axis is just moving.  It still pivots about the CG.  I fear now that we may get the dreaded "comments are not for extended discussion" warning soon, but if you would like to take it up on meta I would be happy to provide another example to illustrate my point.

@MichaelHall But whether I'm doing a coordinated turn, or I just stomp on the rudder pedal out of straight-and-level flight, exactly the same thing happens: the deflection of the rudder produces lift on the vertical stabilizer, which pushes the tail sideways, which makes the nose point in a different direction. You seem to be saying that when I'm in straight-and-level flight and step on the left rudder and thereby make the nose swing left, that's yawing, but if I do exactly the same thing in a coordinated turn, that's yawing. I don't think that's right.

I'm afraid i'm not following you.  Do you know how to take this to chat?


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What is that hole on the front portion of the empennage of a CRJ-1000?
I've noticed that there is a hole on the front tip of the "dorsal fin" of the CRJ-1000. I've been thinking that it would be logical if that's the intake for its APU unit. But I'm not really sure.

Image source: https://en.wikipedia.org/wiki/File:Garuda_Indonesia_Bombardier_CRJ1000_@BPN.jpg

I've added a picture highlighting what I think you must be refering to, but it's worth validating that

Yup thank you for that jamiec

It is the Ram Air Inlet Duct for the cabin air system. It normally acts as cooling air for the packs but can also be used as an alternate vent source in an emergency.

The cooling efficiency of each air conditioning pack is enhanced by
  ram air cooling via a heat exchanger. Ram air enters through an inlet
  at the abse of the vertical stabilizer and passes through the pack
  heat exchangers and then exits overboard. In case of failure of both
  packs, ram air can be used as the sole source of ventilation. This is
  accomplished by opening the ram-air valve via the RAM AIR switch. Ram
  air then enters the occupied areas through the normal vents.

(source)

source: Stack Exchange

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sort: votes


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Why Planes Don't Fly Faster?
I recently Stumbled on this Why Planes Don't Fly Faster on youtube and it talked about how less speed is more economical for airlines and he also said something that plane above and below speed of sound will face less air drag, in the region of speed of sound maximum air drag will be experienced. I am not quite able to understand what he meant there?
Question In the first part :
1.  Why planes fly at M .8?
2.  Why there is less drag above and below M 1 ?.
Also in one of the comment someone said

What generates the most thrust in a turbofan is the bypass air and not the turbine. Usually the fan generates around 80% thrust and the turbine generates around 20%. There are some rare cases where the turbine generates more thrust, but in most high bypass turbofans (which are the most we use today) the fan is what generates most of the thrust.

Question Here : Why turbofans generate most of the thrust with the cold flow?
And that the video is mostly incorrect, can someone guide me here, I am sort of confused.
Lastly : Is that youtube video wrong? , I want to know this because I really liked his way of explaining , and I really want video to be right, but I am confused by the some comments, if one could guide me if video is wrong or right, it would highly appreciated?

@fooot Sir, I want to confirm about that video, if that would not be the case i would have not asked .

@SurajJain I am not sure if I understand your question(s). Could you maybe highlight them in your entry? Are you interested in the factors which have an impact on the economy of an air plane, or are you interested in the thrust-generation in a high-bypass-ratio-jet-engine?

@rul30 Updated.

@RalphJ Sir Could You Help me with that youtube video ?

In addition to everything else, there's a point of diminishing returns.  Even now, for a lot of short to medium distance flights, actual flying takes less time than getting through security, boarding, taxiing, deplaning, &c.  Cut flying time in half, and you spend lots more money on fuel for very little time saved.

The YouTube video is right. There are some inaccuracies, but they are minor and the points made are correct. Does this help? If you need more details, read more of the answers here. Then and only then put a single question up if none of the existing answers were sufficient.

@PeterKämpf Sir, Someone Here Said The Video is disaster , I do not know much about aircraft, that is why i asked , i will read other answers shortly.

What information are you unsure about? Peter already said the video is generally right.

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Was the Coanda effect known in 1868 to Matthew Piers Watt Boulton (the one who described the first ailerons)?
Matthew Piers Watt Boulton is known as the inventor whose 1868 British patent No. 392 nullified any claim of the Wright brothers that they discovered the ailerons (see FIG. 5-7).
It seems now that another invention in the same patent (see FIG. 1) shows an engine similar to one that uses the Coanda effect (see the picture: "Diagram of a generic engine that harnesses the Coandă Effect to generate lift (or forward motion if tilted 90° on its side)" for comparison).
The question is: Did Boulton have really in mind the Coanda effect when he proposed the engine in FIG. 1?
This is the explanation Boulton gave to FIG. 1:
"A body is provided having the form of a solid of revolution tapered at the stern and prow on front. An annular jet of aeriform fluid is caused to issue at its fore part in front of its convexity so as to mix with air and create a mixed stream which flows or sweeps over the convex surface, finally quitting it backwards. In this case the action of the jet causes reduction of pressure on the front and thus produces propulsive effect."
"Figure 1 is a longitudinal section of a body or vessel constructed according to my Invention, propelled by an annular jet issuing at its front. a is the body or vessel containing fluid under pressure and shaped so as to offer little resistance to longitudinal movement in the direction of the arrow e; b is a tapering front or prow connected to a by thin ribs; and c is a cavity so formed with an opening round its periphery that an annular jet of fluid can issue by it perpendicularly to the longitudinal axis of a or nearly so. When the jet so issues it tends to reduce pressure on all sides. By virtue of this reduction of pressure the air (or other medium in which the body is placed) in front being free to move flows backwards towards the jet and mingles with it, while the body a being also free to move, moves forwards in the direction of the arrow e. The jet instead of issuing from c perpendicularly to the longitudinal axis of a may be made to issue at any desired angle by suitably forming the passage round the periphery of c."

FIG. 1 shows an engine that seems to use the Coanda effect and FIG. 5-7 a wing with ailerons (driven by a pendulum but the patent specifies they can also be controlled by hand).
The entire patent of Boulton can be consulted HERE

A diagram of a generic engine that harnesses the Coandă Effect to generate lift (or forward motion if tilted 90° on its side).

I'm voting to close this question as off-topic because it's about history - "what did this individual know," rather than about Aviation as defined in the Help Center.

Regarding the Coanda effect, I do not know if Boulton referred to this phenomenon or had in mind just the law of Bernoulli and possible other things. Regarding the ailerons, it is clear from his patent (see the explanations from here: https://en.wikipedia.org/wiki/Aileron) that he proposed ailerons and nothing else for controlling the longitudinal stability of what he called "aerial vessels". He even talked about controlling the flying machine in the vertical and horizontal planes by other means. He was fully aware of three-axes control.

I voted to leave this open...I would be interested in hearing the answer.  Also, why would there be an aviation-history tag if history questions were off topic?

Also, @simplex can you provide evidence that the wright brother claimed to have invented the aileron?  I was unable to find any evidence of this.  The wright flyer, the first "airplane" did not have ailerons but instead used wing warping for roll control.  Otherwise, please reword your post.

@bclarkreston The Wright brothers claimed they had invented not only the wing twisting method for stabilizing a plane in roll but the ailerons in general. See the paragraph titled "Patents and lawsuits" of this page https://en.wikipedia.org/wiki/Aileron.

I found a similar device described among the papers of pioneer aeronaut J.W. Dunne (held in the Science Museum archive centre). He drew in the working fluid via a mirror-symmetrical arrangement at the back and pumped it round via a propeller in a central duct. The idea appears to date from around 1901, so now I have to decipher Dunne's handwriting properly to discover whether he knew about Bolton's patent yet! (as an aside, in his defence against the Wrights, Dunne referenced an aileron patent by Harte rather than Boulton)

The accompanying text clearly describes an application of the Bernoulli effect. However the resultant backwards airflow shown in the drawing is given no explanation, other than that implied by the fact that the device is drawn forwards. So there is no evidence of the Coanda effect, in which the airstream clings to the device skin, being understood.
Would it have worked? Probably, but it would have been pretty inefficient.


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What design feature prevents the fuselage skin from completely tearing off?
For an example, I am talking about the Southwest Airlines Flight 812 where there was a small tear in the fuselage section caused as a result of metal fatigue.

Why didn't the tear affect other sections of the fuselage?
Is there any specific design feature involved to prevent such a situation or is it just plain luck that it did not happen?

In the 1980's, manufacturers thought that they had solved the problems that plagued earlier models like the De Havilland Comet. Then Aloha Airlines Flight 243 had a large section of upper fuselage come off during flight. Manufacturers realized that the fuselage could accumulate more damage more quickly than expected, and when this damage went undetected, it resulted in catastrophic failure.
The result was both regulators and manufacturers emphasizing a focus on both damage tolerance and fail safety when designing these structures.
Damage tolerance means that the structure can tolerate a certain amount of damage while still performing its function. Inspections should be performed soon enough that they will detect the damage before it compromises the structure.
Fail safety means that even if a feature fails, the overall structure can still perform its function. One option for this is redundancy. In the fuselage, this means stopping cracks from propagating too far. The cracks in the Aloha Airlines incident were able to continue growing until a large portion of structure failed.
These ideas mean that the structure is designed so that any damage is caught before it becomes critical, but in the event that this is not sufficient, items can fail without endangering the safety of flight.
An example is what happened to Southwest Flight 812. A section of fuselage ripped open in flight, which resulted in a rapid depressurization but a safe landing. A crack formed in the fuselage and was not detected. However, the crack was contained in a limited section of the skin. This allowed the cracked skin to fail while the rest remained intact.

Aloha Airlines flight 243 already had the structure designed to stop propagation of structural failure. There is a theory that the initial hole got blocked and that caused pressure spike that destroyed more of the skin, but because the torn part was not found, it was not possible to either confirm or disprove that theory.

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What is airport tax used for?
Since a very large sum of your ticket goes to airport tax, I was wondering how do they justify it and what is it used for?
Take your tax paid on your ticket and multiply it with the number of seats on that airplane and you realise that, per flight, the airports are receiving a substantial amount.
Is it all going to the government?
This question is very 'general' and just to get a loose idea of what the tax goes for. To get detailed information of every country is unnecessarily complex.

While I do believe the airport receives a substantial amount (fly into Frankfurt as a private pilot, and you will feel the pain), doesn't the 'tax' portion go to some kind of government?

running an airport isn't cheep so they need to recoup costs somehow, overcharging for other things doesn't cut it

@ratchetfreak but is that the 'tax' though?

How about Air traffic control? I can imagine that will cost quit a lot of money.

My guess would be that what airport tax money is used for varies considerably depending on which airport in what country is being talked about. Also, what all does one consider a tax. Are landing fees a tax, are fuel surcharges taxes, are bribes to local officials in third world countries to be considered a tax?

@roe maybe indirectly. Directly it goes to the airline...

As @Terry points out, this can not be answered without knowing what part of the world you are asking about.  Please edit your question to let us know what country you are asking about.

@voretaq7 Why did you edit this to add the "usa" tag? The asker gave no indication that they meant to restrict the question to any specific country.

@DavidRicherby As has been pointed out in the other comments this question cannot be reasonably answered without the context of a country (because tax is a local/regional thing). The the accepted answer is in the context of the USA so I feel reasonably sure that was the asker's intent.

Assuming the question was referring to the US airport market, where indeed a big chunk of the airport fees are indeed taxes, here's their composition according to Airlines for America:

As jwenting pointed out, there isn't one "airport tax" - there's a whole bunch of them, and they're not all taxes (some are "fees").
airlines.org has a list of the common ones you'll pay in the USA, and approximately what they cost you.

Broadly the taxes you're paying on airline tickets break down into two categories:
Taxes that fund the Airport and Airway Trust Fund
These taxes pay for Air Traffic services (a little under half the operating butdget for ATC), maintenance of the airway infrastructure (radio navigational aids, RADAR sites, etc.), research and development to improve the airway infrastructure (e.g. NextGen), and to fund the Airport Improvement Program (which provides grants to airports, mainly to implement safety improvements).
The actual tax categories themselves break down further (again, airlines.org's breakdown is excellent).

Fees that fund DHS programs
These are often called "security taxes", and include things like the "September 11th fee", APHIS fees (for inspecting plants and livestock), and customs/immigration charges.
Note that all of these are technically fees, not taxes -- these are only levied when a flight is using the services the fee is funding (or at least that's what's supposed to happen. In practice the airline is responsible for paying the appropriate fee as a cost of doing business. They can recoup that cost any way they'd like (typically they lump it in with "taxes" or itemize it out on the ticket rather than including it in the base fare, so they can advertise lower base fares).

What about the little planes?
General aviation doesn't pay "ticket taxes", but that doesn't mean they get to escape paying for the infrastructure the flights are using. The government recognizes that the air traffic system costs money, and they like to ensure that the money is available.
Accordingly there is a tax levied on all aviation fuel purchased in the US:

Airlines currently pay about $0.04/gallon in taxes for the jet fuel they burn
Non-Airline flights burning jet fuel pay about $0.22/gallon in taxes
(the higher rate making up for the other taxes they're not paying)
Piston-powered planes burning aviation gasoline pay $0.19/gallon in taxes
(higher than what the airlines pay for jet fuel, but lower than the standard jet fuel tax because piston planes don't travel as far, and are thus less of a burden on the air transport system)

Also note that airport taxes aren't the only fees you'll pay - many airports have landing fees, ramp fees, terminal fees, etc. which are levied by the local airport authority / operating agency. For example, The Port Authority of New York and New Jersey has a published schedule of fees and charges for aircraft landing at JFK. These fees go to the Port Authority

Note that, in the UK, it's illegal to advertise a fare that doesn't include all the taxes, fees and other things you can't avoid paying if you want to buy the ticket. Various budget airlines have a habit of getting into trouble, in this respect.

@DavidRicherby If only the US would adopt such sensible advertising laws. Around here very few advertised prices include taxes (and many don't include fees either). SURPRISE! :-/

Actually, they do.  The DOT passed a law last year  requiring all taxes and mandatory fees to be included in the advertised ticket prices.

I didn't think the US did this either, but just confirmed it with four different carriers. All taxes and airport fees are now included in the advertised price, and baggage and other fees are prominently linked. Very cool.

There are many things which together make up the 'tax' listed on your ticket. And depending on the airline this may or may not be honest; many use the term to bulk up the final price from the list price (and then advertise the list price of course). For example, a "fuel tax" is a sum the airline adds to counter changes in fuel price from the moment the ticket price was set to the moment you board the flight. And the "security tax", were that really a tax, then why do different airlines charge different amounts for the same departure/destination pair? There might be a tax component in it, but it's massively padded by the airline itself (and most of the rest will be not tax, but contract fees with their contractors).
Of course some countries do charge taxes, but those are usually collected directly and not as part of the ticket.

Airports may be managed by private entities (even private security agencies) and then airports themselves are expensive to maintain, so apart from leasing its space for shops/eateries etc, they charge every passenger a tax which they collect from each airline. Its the duty of the airline to collect taxes and reimburse it to the concerned authority (whether private or government but probably government).
But all "taxes" on the ticket are not taxes; as jwenting pointed out, the yq (or yr) component on the ticket is actually a "tax" levied by airlines at their discretion and is called fuel surcharge. Other "tax" components may be an ROE (Rate Of Exchange) component which helps airline absorb fluctuation in currency (if they are operating in the country having different currency from theirs).

If it is genuinely a tax, it goes to the government. Other countries may be different but, in the UK, all money that the government receives from taxes goes to a central fund to be spent wherever it's needed: there's no mechanism for, say, the money from aviation-related taxes to be reserved for aviation-related spending, such as ATC.
Fees and surcharges are different and may go to the airport, the airline or whoever else charged them.

In the US, you can see most airports income, by going to the FAA site: http://cats.airports.faa.gov/Reports/reports.cfm. This lets you see the breakdown by year, and revenue sources and costs. Form 127, contains the most detail in the data available there. Running, modifying, and just maintaining an airport, is very expensive.


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Can an automatic vane-based stabilizer really work?
Can the pitch stabilizer in the picture really work for a plane or glider?
The image shows an automatic pitch stabilizer (the mechanism in the black rectangle) that is based on the vane 50 whose movements up and down would drive, with the help of a force amplifier system, the horizontal front elevator of a Wright glider or plane in such a way as to maintain the aeroplane in horizontal flight.
More precisely if, for a reason or another, the plane have the tendency to dive there will appear a vertical component of the relative wind that will push the vane 50 upward and, through the mechanism already mentioned, the vane will drive the front rudder in such a way as to make the apparatus climb again. If the plane tend to rise, the vane will go down and command the front rudder to make the plane descend.
The question is, can this mechanism really work, at least theoretically? (A stabilizer based on a pendulum is a fallacy, it will not work. In an accelerated system (the plane) the pendulum does not align to the vertical).

Source: US patent No. 1075533 (filed by WRIGHT Co in 1908)
(I am not interested in answers regarding the automatic stabilizer in roll, also described in the patent. It is based on a pendulum and it can not work. My question is only about the pitch stabilizer.)
UPDATE:
My question has nothing to do with a pendulum stabilizer. The vane-based stabilizer (the subject of my question) works on a completely different principle. In consequence, the answers given to this question, that refer strictly to a pendulum stabilizer, do not help.

@RobertDiGiovanni Yet somehow this guy seems think pendulous vanes actually prove the earth is flat. He seems to know his stuff yet his delusion persists.

Yes, unless he is seeing how many people will fall for his reasoning.  The earths rotation may be too slow to overcome friction required to move the gyro, but laser attitude indicators are worth checking out.

This is basically FBW works on every jet fighter after F16: introduce a negative feedback on the pitch axis to add pitch stability, although a pure mechanical implementation.

@user3528438 As described, this system would introduce positive feedback. A steep angle of attack (descending or nose high) would result in up elevator, and a shallow angle of attack (climbing or nose low) would result in down elevator. Or maybe I'm misunderstanding something.

Actually combining a pitch controller and a pendulum will work a bit better, especially if the throttle is constant.

Yes, this vane-based stabilizer really did work. In fact the pendulum system for direction control also worked as it was used to sense yaw in a slip, just like the ball in a turn and bank indicator.
The entire system worked so well that Orville Wright received the 1913 Collier Trophy "For development of the automatic stabilizer" after demonstrating it for the judges.
The Wright Bros work in automatically controlled flight was overshadowed the following year when Elmer A. Sperry won the 1913 Collier Trophy "For gyroscopic control".
WRIGHTS DEVELOP AUTOMATIC STABILIZER

In the fall of 1913, Orville installed the stabilizer on a special Wright Model E airplane that utilized a single pusher propeller. He kept the details of the stabilizer secret even from the Wright Company. He purposely waited until the last day of the year to fly for the prize.
He invited the Aero Club’s judges to Huffman Prairie to see a demonstration of his new device on a cold snowy day, December 31st.
He turned up his coat collar, put on a pair of goggles and took off. He made a total of 17 flights.
His most spectacular flight consisted of 7 full circles of the field with both hands held in the air. The automatic stabilizer kept the same angle of bank and almost the same altitude. He wowed the judges and was awarded the prize on February 5, 1914.
The stabilizer was then offered as an option for use with the sale of Wright 1910-1911 Model B flyers.
However, it saw little use, because on June 18, 1914, a young Lawrence Sperry, as part of a great safety competition, unveiled a totally new type of stabilizer to the world. The safety competition was jointly sponsored by the Aero-Club de France and the French War Department.
In his demonstration flight, Sperry took off from the Seine in a Curtiss C-2, climbed to altitude and flew back down the river. At the appropriate moment, his mechanic, Emile Cachin, crawled 7 feet out on the wing as Sperry lifted his hands from the controls and stood up in the cockpit. The plane sped by the judges as the crowd went wild.
What Sperry had done was adapt a balancing mechanism invented by his father, Elmer, for counteracting the rolling of ships, to an airplane. The device employed two gyroscopes that performed the function of the pendulum and vanes in the Wrights’ device.

Mike +1. In fact, the pendulum would prevent an AOA only system from being "fooled" by relative wind only inputs.  Pendulums are not "fallacies", as you pointed out, they are turn (and gravity) acceleration force indicators.  Combining that with a relative wind indicator would allow one to SET and HOLD an attitude. But, ah yes, the Sperry gyroscopes eclipsed the Wright patent.  Really a +10!

The Wrights could've achieved the same end by incorporating an anti-servo tab on the canard surface, as is used on stabilator tails (invented by John Thorp in the 50s) for which the tab is the primary source of static pitch stability.

See this demonstration: Drone Pendulum Fallacy (https://www.youtube.com/watch?v=OYHCP3-mpxk). Therefore, a pendulum can not stabilize a flying machine.

The article: Wrigh Stabilizer Greatly Refined ( https://chroniclingamerica.loc.gov/lccn/sn84026749/1914-01-05/ed-1/seq-5/ ), Washington Times, January 5, 1914, talks about Orville Wright as taking a prize after flying in a circuit a few times with his hands removed from the controls but the 1913 Wright planes were naturally stable and there is no guarantee that Orville did not drive the plane by moving his seat (with his body), as the Wrights did in 1902 with their glider.

A pendulum will move from its vertical position if velocity is changed.  This movement can activate a control surface to create force in the other direction.  A pendulum can be used to MAINTAIN a given attitude, as Wright did in flying a circle at constant speed.  The pendulum would swing outside the turn, and remain there until the aerodynamic turning forces were removed.  Importantly, it would then return to its original position.  In steady state flight, the pendulum, like the pilot, experiences 1 G towards earth (straight down).

Remember, the pendulum in the control device did have a fulcrum.

Based on all comments the question received, I will try to formulate a clear answer illustrated with a picture.
The vane V will always tend to stay in the same plane as the axis XX' no matter what the glider does: climbs, descends or flies horizontally. If the vane goes toward the upper or lower wing then automatically it will drive the mechanism that changes the angle of the front rudder till the air current pushes the vane in the opposite direction and so on. In consequence, there is no way to predict which direction the plane will align to if a perturbation changed its initial direction of flight.
As a remark, the weight E together with the weight of ED just counterbalance the weights of DC, CB, BA and that of the vane V that is bolted to BC. The sum of momentums of all these weights with respect to pivot D is always zero. The weight E will not tend to skew the parallelogram ABCD in any way.

Top: Glider descending. Bottom Glider in horizontal flight. In both cases the position of the vane V is the same.

"Vane will tend to stay in the same place", yes that's how it works. The Wright system could only be set for one condition.  It would respond to a "perturbation" to return it to its original condition, but, unlike a gyro attitude indicator, could not provide information as to where the original attitude was, there for the pilot was "out of the loop" if additional changes were desired.

No, I don't see how this would work as a controller for maintaining flight level. The angle of attack on any part of the airplane during a steady-state climb is identical to that of level flight, so there is no additional "relative wind" for the mechanism to work with.
However, a proportional feedback in angle of attack is a valid stability augmentation system. With the right gain, it has the same effect as adding positive static margin. The net effect is maintaining airspeed, which, given the same power input, would also contribute to flight level maintenance. This was a success for the Wrights' probably because its first flyer was statically unstable.
Positioned correctly, it may also serve as a pitch damper. But once statically stable aircraft prevailed, it's likely this was superseded by gyroscopically measured pitch dampers as mentioned in Mike Sowson's answer.

The pendulum, as well as your senses, work correctly in steady state flight (1 G straight down) and will "hold its place", albeit not "straight down" in a constant rate/constant speed turn.  Gyros "know" where the horizon is, but even they can accumulate error.  Sadly, our senses, like the pendulum, can get thoroughly lost after a few turns and attempted corrections.

This appears to be a mechanical version of the modern computer driven Angle of Attack vane / Stabilator pitch control system.  The catch is it is based on relative wind.  A plane can do an entire loop without changing its AOA with a very good pilot using throttle and gravity.
So, back in 1908, to maintain altitude, one might try a balloon mounted in a container, with a mechanical lever and spring attached to the carburetor. If the balloon expanded from greater altitude, the lever would lower fuel input, if the balloon contracted, the spring would pull the lever back to increase the fuel input.  This could also come with disclaimers regarding changes in pressure due to weather.
Setting a plane up with weight slightly forward of the main wing Center of Pressure (Lift) and conventional tail downforce, or forward canard upforce (staticly stable), will do a pretty good job maintaining speed and AOA.  Adding or subtracting throttle would make the plane climb or descend at the same speed to correct an altitude deviation.
It is interesting to note, though many hail the Wrights early canard designs, their first commercial success, the Model B, featured a rear mounted horizontal stabilizer and flew with an engine power output of 30-40 horsepower, enough to carry a passenger.

While the AoA vane is hinged at its quarter chord, the Wright vane is hinged at mid chord. This makes the AoA vane indifferent and the Wright vane highly unstable.

@Peter Kampf.  Absolutely, and your writings explaining aerodynamic center are certainly good reading

@Robert DiGiovanni, I am not sure I understand your reasoning. I guess you are aware that "A Pendulum on a Merry-go-round" ( see: http://physicstasks.eu/1753/a-pendulum-on-a-merry-go-round ) will align to a direction making an angle alfa to the horizontal (after the oscillations die). So, if a Wright plane had flown in a circle in a perfectly stable way then the pendulum onboard would have made an angle alfa (dependent of the angular speed of the plane and the ray of the circle) to the vertical. There would have been no alignment to the vertical in the steady state regime.

@simplex correct!    "after the oscillations die" the pendulum would have made an angle alfa dependent of the angular speed...  AND GRAVITY!  Gravity still contributes to the direction of the pendulum AND the pendulum reaches a steady state!  So, the Wrights "trick" is to enter the turn FIRST, then set the control device, then hands free. This is also why the "ball" is not perfectly centered in a steep turn, but works well to coordinate shallow turns.  And even gyros need an "self erecting"  pendulum to help remove accumulated errors from abrupt maneuvering.

@Robert DiGiovanni, The acceleration of the suspension point of a pendulum inside a plane is chaotic in general because the plane continuously tends to roll to the left or to the right, pitch down or up and turn around its vertical axis. In consequence, the pendulum tends to align to a direction that is dependent of g + acc_chaotic = acc1_chaotic (all of them vectors). In other words, the pendulum at each time tends to align to a chaotic direction and so it never aligns to anything. How can this chaotic oscillator be useful for stabilizing the flying machine?!

@simplex Well, first we'll make sure the string holding the pendulum has minimal elasticity.  I chuckle because I tried this with my CB mike, which has a coiled cord.  Once constant rate and speed turn is established, the pendulum will "settle". Only then, as you point out, is it useful to hold course, allowing Wright to dazzle with "hands off" circular flight.


text:
Did the Wright brothers discover the three-axis control system for airplanes/gliders and the adverse yaw?
1) Various sites like this one: Orville and Wilbur Wright, The Inventors of the 3-axis Flight Control System, 9 Months before their powered flight at Kitty Hawk say that the two brothers invented the three-axis control system.
Is it true?
This is what they wrote in the beginning of their British patent: 1904-03-19 – 1904-05-12, Orville and Wilbur Wright, “GB Patent No. 6732, A.D. 1904 – Improvements in Aeronautical Machines”, Date of Application: 19th Mar., 1904, Accepted: 12th May, 1904, 5 pages, the first that was granted to them.

"Our invention relates to improvements in that class of aeronautical
  machines in which the weight is sustained by the reactions resulting
  when thin surfaces, or wings, are moved horizontally almost edgewise
  through the air at a small angle of incidence, either by the
  application of mechanical power, or by the utilization of the force of
  gravity.
The objects of our invention are, first, to provide a structure
  combining lightness, strength, convenience of construction, and the
  least possible edge resistance; second, to provide means for
  maintaining or restoring the equilibrium of the apparatus; and third,
  to provide efficient means of guiding the machine in both vertical and horizontal directions."

The expression "three-axis control" does not appear in the patent.
Regarding this patent, more precisely its German version, filed on March 23, 1904, (four days after the British text) the fact that the wing twisting and the vertical rudder had nothing to do with steering the machine was clearly explained by Harry Aubrey Toulmin (the Wright brothers’ patent lawyer) to Carl Pieper (his German correspondent) in a letter dated April 11, 1905. Here is the relevant excerpt of this document:

“It is evident from the action of the German Patent Office that the
  invention is not yet understood by them. The entire purpose of the
  structure on which the application is based, excepting the front
  rudder, which is for another purpose, is to maintain the body of the
  structure level or parallel with the horizon. It has nothing
  whatever to do with steering, neither has it anything to do with the
  raising and lowering of the plane of flight. The setting of the two
  parallel edges of the supporting planes at different angles is solely
  for the purpose of maintaining the wide body of the machine parallel
  with the horizon. The rear rudder is solely for the purpose of
  overcoming the tendency to turn around a vertical axis which naturally
  results from setting the two edges of the planes at different
  angles. … We shall trust to you to place these matters before the
  German Office in the best way possible and hope that you will be
  successful in obtaining the allowance of the application.”

H. A. Toulmin, “Letter to the Carl Pieper”, Springfield, Ohio, April 11, 1905
The explanation above is puzzling and seems to nullify any kind of pretentious that the Wrights invented the three-axis control system as long as their patent application had “nothing whatever to do with steering”.
2) Regarding the adverse yaw, the description given in the patent and the explanation of Toulmin are quite clear. The Wrights proposed a method for counteracting this unwanted effect but where they the first who noticed it?

The Wright brothers glider as it appeared in the US patent granted on May 22, 1906, which is a bit more detailed than the drawing in the British patent, granted two years before.

yea because i know
https://aviation.stackexchange.com/questions/73054/did-the-wright-brothers-discover-the-three-axis-control-system-for-airplanes-gli

That Toulmin guy obviously had no idea what he was talking about!  The Wrights didn't even include a moveable rudder at all till partway through their 1902 experiments.  Obviously they concieved of banking as the way to steer.  I need to update my answer to emphasize this point.

The patent lawyer is explaining things quite correctly and this applies to modern aircraft.  The rudder does in fact have nothing to do with steering the airplane.  Its job is to keep the tail lined up behind the nose (or to move the tail out of alignment with the nose if you wish) mainly overcoming adverse yaw caused by warping (or later, ailerons).
The bolded text in the paragraph you quoted describes the wing warping being for roll control, and the rudder for controlling for the adverse yaw that results from the wing warping (same as aileron displacement in modern aircraft), just like any modern aircraft.  He is describing how a modern airplane flies in other words, and this was a pretty radical idea at the time so it didn't make sense to a lot of people.
The Wrights discovered adverse yaw as they were developing their totally new wing warping system concept, as it was abundantly obvious from the way the nose would slew the opposite direction to the wing warp input and a rudder control was the obvious solution.  They didn't get it from anybody else because nobody else had thought of the concept of differential lift to control bank in order to turn.   Lillienthal, from whom the Wrights based a lot of their work in the beginning, used weight shift to control his gliders.
I recall from books I read on the Wrights years ago that they proceeded originally from from Otto Lillienthal's aerodynamic theories, and as they tested their glider machines, they started to discover that Otto had a lot of things wrong, prompting them to build their home made wind tunnel and start development from scratch.
So I would say for (1) yes, they invented the 3 axis control system, that is, all 3 being used together to make coordinated banked turns, because they developed and patented wing warping (ailerons coming later and leading to a long patent legal battle with Glenn Curtiss).  Up until then, and for some time after, developers were controlling turns by skidding using rudders (helped by dihedral effect to achieve banking in some cases) or by weight shift, or in Bleriot's case trying to achive roll control with differential elevator movement like an F-15 to get around the Wrights' wing warp patent (which they defended furiously, leading to the long patent battle with Curtiss, who ironically ended up taking them over in later years and becoming Curtiss-Wright, a running concern to this day).
The proof of this is obvious if you read of the various accounts describing the Flyer's demonstrations through the mid 00s.  The Flyer's contemporaries, such as they were, would slither about the sky, whereas the Flyer flew around making coordinated banked turns like a modern aircraft.  This astounded observers, especially the French when the Wrights demonstrated the Flyer in France.  The French reaction to the Flyer was really something else, one of awe, admiration, and a bit of jealousy, that those two American yahoos brought a flying machine that could literally fly circles around machines like 14-bis.
So for (2) I would say the answer is also yes because they invented differential lift for roll control in order to turn, which quickly indicated the need for a rudder to counteract adverse yaw and which followed on naturally.

If the vertical rudder plus ailerons (wing warping) are not used for steering, then they are just for stabilizing the flying machine in roll, which would be one-axis stability. The patent would by about the stability about the longitudinal axis and control/stability about the transverse axis (pitch). The yaw control (steering the plane to the left and to the right) is not covered and in consequence the patent is not about 3-axis control. It appears that the Wrights were simply not allowed to claim steering (in pitch and mainly yaw) because the two movements were covered by other patents.

The July 15, 1908, issue of L’Aérophile (https://gallica.bnf.fr/ark:/12148/bpt6k6550620m/f284), after saying that a Blériot VIII had flown for 8 min and 24 sec on July 6, 1908, a notable performance for a monoplane, noticed the impressive manœuvrability of the apparatus, due, in part, to its ailerons (not based on wing warping). This flight took place more than 1 month before August 8, 1908, when the Wrights first showed their machine on the ground and in flight. That day Wilbur flew for 1 min and 45 sec. As a remark, on July 6, 1908, Farman flew for 20 min 20 sec without ailerons.

Wright's patent was only for wing warping for roll.  I never said they patented 3 axis control; I just said they were first to apply it in the modern sense insofar as they introduced the 3rd element, roll control, and they used the rudder for it's correct purpose in the modern sense, to keep the tail lined up behind the nose.

There were already at least 2 methods for controlling the roll, one by moving weights along the wings and the other, more practical and proved in practice, using dihedral stability. A. Graham Bell made a statement (see: https://chroniclingamerica.loc.gov/lccn/sn95079481/1896-05-13/ed-1/seq-8), in May 1896, declaring himself impressed by the stability of Langley's model powered plane both when the apparatus climbed as a plane and when it descended as a glider. Langley's plane had automatic roll stability. The Wright gliders had means (ailerons) for manually stabilizing the machine in roll.

You said: "they [the Wrights] used the rudder for it's correct purpose in the modern sense, to keep the tail lined up behind the nose". I do not really get the meaning of this statement (I do not say it is wrong, I just say I do not understand it). On January 13, 1908, Henri Farman made turns, without ailerons, and flew 1 km in a circuit at a stable altitude. He steered the airplane, in the horizontal plane, just with its tail (vertical rudder).

What is still unclear for me is whether there is a difference between "roll stability" and "roll control". I saw that some planes can fly in a straight line with their transverse axis (wing tip to wing tip) making a stable angle, say 30 deg or 45 deg, with the horizontal plane. Is this standard roll control or just a complicated maneuver done by stunt pilots?

I think you need to educate yourself on how airplanes fly. You turn by orienting the lift vector at an angle - by banking.  You CAN turn by skidding laterally, like a car drifting,  but it is very inefficient energy wise and unpleasant for the persons on board.  Roll stability isn't the issue here.  It's control.  The Flyer was no particularly stable in any axis, but had unrivaled control compared to its contemporaries getting by without wing warp or ailerons on the main wing.  People who observed the Flyer, after having seen its competitors, were astounded.

Airplanes use dihedral for roll stability, which requires a little bit of sideslip to be present to do its thing.  The vertical fin has to be sized to allow just that little bit of sideslip, once a bump induces a bit of spontaneous bank, before weathervaning the airplane into wind.  Weathervane too soon or strongly and the airplane spirals instead of righting itself.  Now, in the absence of ailerons, you can use a lot of dihedral to generate a strong roll with skid, as early aircraft did, and modern 2 axis ultralights like the Quicksilver did.  But it's a very sub optimal solution.

@simplex read the link I added to the post about France.

What I know is that the Wright gliders were terrible unstable, unlike those made by Lilienthal or Chanute. Finally, starting with August 8, 1908, when Wilbur was first seen flying an airplane, the plane he used had dihedral stability. The Warping was used in parallel with the dihedral but the plane could have made turns without its ailerons. I read the articles in French from August 1908 (there are more than 100). There was a journalist form L'Auto who advertised the Wright planes but the general opinion did not favour their machines, they were hard to drive and unable to take off unaided.

But they MANEUVERED like a modern aircraft, which was unprecidented. Stability is not the issue.  You're not really getting it.

This is the plane used by Bleriot (see: https://gallica.bnf.fr/ark:/12148/bpt6k6550620m/f264.item.r=ailerons) in its July 6, 1908, flight of 8 min and 24 sec about which I talked before. Its modern ailerons are visible.

Text referring to the July 6, 1908 flight: "The Blériot VIII often altered, according to what the practice required, maneuvers with remarkable ease. The two ailerons which serve as elevators and, if moved in opposite directions, help in making turns, seem to give results at least as good as those promised by the Wrights with their wing twisting device; all steering, by an ingenious patented arrangement, is controlled with a single lever. The ailerons make flight possible in turbulent wind." (L'Aerophile, July 15, 1908, see: https://gallica.bnf.fr/ark:/12148/bpt6k6550620m/f284)

If you think the Wrights were the first to invent wing warping, please research why they could never get it patented in Germany. You will be surprised.

@PeterKämpf I realize that things are usually invented by various people working independently and somebody is the lucky winner.  do you have a link of some kind?  I'd be interested to read it.

Sorry, no link. Someone discovered a bee with a deformed wing on one side which could only fly in circles and took that as the inspiration to patent wing warping in Germany. Also, Lilienthal exchanged sketches of wing warping mechanisms with other aviation pioneers.

The Wright brothers were not aware a glider or plane could be steered to the left or to right by rolling it to the left or to the right
The two inventors did not discover the method of turning a plane by banking it to one side or the other.
In their patents, what they had in mind was the classical steering with the vertical tail rudder (not invented by them) while the wings of the airplane were maintained as parallel to the ground as possible by stabilizing the flying apparatus in roll using the Wing Warping method.
It is a myth that the two brothers from Dayton realized a plane could be made to take turns by banking it.
They have two more patents, filed on February 17, 1908, (see the drawings below) in which the tail rudder is disconnected from the wing warping mechanism, the tail being free to move independently of the twist of the wings. The adverse yaw, unlike in the May 12, 1904, British patent, is corrected with two vertical vanes placed close to the wing tips in the US Patent No. 1,122,348 and with another mobile vertical rudder placed in front of the main wings in No. 987,662.
None of these two patents nor the May 12, 1904 British patent talk about steering by banking.

Orville and Wilbur Wright, “US Patent No. 1,122,348 – Flying-Machine”, Filed: February 17, 1908, Patented: December 29, 1914

Orville and Wilbur Wright, “US Patent No. 987,662 – Flying-Machine”, Filed: February 17, 1908, Patented: March 21, 1911.

Well they sure are banking it to turn in this 1909 film https://www.youtube.com/watch?v=dtZ8MxuePno.

I read through the patent linked and nowhere does it say they turn by skidding.  The vertical vanes described are for control of "secondary effects" of the wing warp - adverse yaw.  They also turned their 1902 glider by banking.  They had to; a glider with no thrust source beside gravity and no side area to speak of will simply not change heading fast enough to be useful if you skid it.  It will just carry on more or less in the same direction  with maybe a painfully slow heading change.  Here is a pic of the glider turning https://www.britannica.com/topic/Wright-glider-of-1902.

That 1909 plane had dihedral stability. The gliders in the 1904 British patent and the two US patents of February 17, 1908, did not have automatic stability in roll. If the airplane is naturally stable, and it also possesses ailerons, just moving the ailerons in opposite directions will determine it to start steering by banking. The Wright brothers might have noticed this effect starting with August 1908.

In their patents, nowhere it is written that the Wrights intended to turn their airplane by skidding or banking. In the Feb 17, 1908, US patents both steering by skidding and banking are possible but the two inventors do not say how the machine could take turns. There is no word about the lateral force that appears when the plane banks.

It is clear now that the Wright brothers had in mind skidding not banking because this is what they said in their Feb. 10, 1908, patent about automatic stabilization (see: https://docs.google.com/viewer?url=patentimages.storage.googleapis.com/pdfs/US1075533.pdf): “If it is desired to drive the machine in a circle ... set the vertical rudder at an angle to its normal position, and, with the parts thus reset, the automatic-controlling mechanism will operate then in exactly the same manner as when the machine is being driven forward in a straight line”.

No, the Wright brothers did not invent the three-axis control, at least in their Patent No. 6732, A.D. 1904 (They might have other patents which cover: roll, pith, and yaw control together. I do not speak for them if they exist.)
In No. 6732 the two brothers do not have a clear explanation about steering the glider in the horizontal plane. The Wrights do not say how they control the yaw rotation (except the adverse yaw) and in consequence the patent does not make them the inventors of the three-axis control system.
What the patent explains well is:
1) the stability in pitch and also to a lesser degree but still understandable the control of the pitch angle (the steering in the vertical plane).
2) the stability in roll and how the adverse yaw is compensated.
Their claim:

to provide efficient means of guiding the machine in both vertical and
  horizontal directions.

is not sustained, regarding the horizontal direction, by the explanations they gave.
With reference to the image below, we will assume that at moment t0 the plane was flying horizontally with its wings parallel to the ground and the pilot attempts to steer it to the right by rising the tips of the left wings and lowering those of the right wings. As the rear vertical rudder is interconnected with the wing warping mechanism and the angle it makes, to the neutral position, can not be independently adjusted regardless the level of the twist, the rear rudder will automatically turn to the right to compensate for the adverse yaw that rotates the plane to the left. If the pilot forces the configuration "Left wing tip up, Right wing tip down, vertical tail to the right" he can no longer control the roll stability. The plane will turn to the right only assuming that during this maneuver it does not have any tendency to roll chaotically due to its inherent instability in roll, which is not the case.

How the two brothers maintained their glider stable while taking a turn remains an unexplained thing.

While taking a turn to the right, the Wright brothers might have thought of keeping the vertical rudder at a reference angle, say 45 deg, to the right and balancing the plane by moving the rudder a few degree above or below this reference angle. However they do not say this. They do not detail how the vertical rudder in synchronization with the wing twist can steer the plane.

What they had was the Ercoupe's no-rudder-pedal control system, where the rudder was geared directly to the ailerons and worked only to compensate for adverse yaw induced by wing warp.  It's blindingly obvious to anybody who flies that they could not have turned their glider without banking it.  A glider with no side area and no engine will simple slither sideways if you tried to "steer" it with rudder (you are actually skidding it).  If you get any turn rate out of it, it will be too slow to be of any utility.

In their Feb. 10, 1908, patent they explain how the plane could be steered by skidding just by turning the vertical rudder to one side (applying a shift, setting a new zero for its position) and then working the rudder and ailerons exactly like during a flight in a straight line.

The vertical rudder appears to be interconnected to the wing warp mechanism so a rudder input results in a roll not just a skid sideways.  That kind of machine skidding sideways will take a week to effect a change in direction. If the film of it you can see it is banking in all its turns.  Don't put too much into the patent language.  The patent also describes a pendulum/air cylinder driven servo contraption for creating artificial stability which wasn't implemented.

In the Feb 10 patent the Wrights imagined a special mechanism that allowed them to set by hand the initial position of the vertical rudder, independently of the wing twist, and then they coupled the rudder to the automatic mechanism which moved it relative to its new position. Read my previous comment that contains the text "If it is desired to drive the machine in a circle" for more explanations. 2) Orville said in 1913/14 the Feb 10 patent had been implemented and it worked!!? (see: https://aviation.stackexchange.com/questions/71934/can-an-automatic-vane-based-stabilizer-really-work).

All films showing a Wright machine taking turns and flying, in general, were made after August 8, 1908, and the planes shown there were stabilized in roll by the dihedral method, not covered by the Wright patents. The simple action of twisting the wings would have turned those airplanes by banking. The problem is that the Wrights do not talk about this effect, turning by banking, in their patents. So they did not discover it.

"1) Various sites like this one: Orville and Wilbur Wright, The Inventors of the 3-axis Flight Control System, 9
   Months before their powered flight at Kitty Hawk say that the two brothers invented the three-axis control system.
Is it true?"

Matthew Piers Boulton proposed the combined use of ailerons, a pitch control surface, and a rudder well before the Wrights began their experiments.
However, the Wrights were the first to actually demonstrate a working 3-axis control system, and came up with their control system largely through their own independent thought plus actual trial and error.  They also appear to have been the first to combine an understanding that banking was the most efficient way to turn an aircraft, with the design of an effective system to accomplish a change in bank angle,  with an understanding that a movable rudder was an effective way to counteract adverse yaw but should not be used as the primary "turning" control.  So if you are asking if the Wrights were the first to come up with something resembling the modern understanding of how a three-axis control system should be used to maneuver an aircraft in flight, I would say "yes".   In aswering this one shouldn't rely too heavily on what the Wrights did or did not choose to include in any given patent application, and one certainly shouldn't rely on a lawyer's attempted "explanation" of the content of any given patent application.
On the other hand if you are asking if the Wrights were the first to ever consider the combined use of pitch, yaw, and roll controls on an aircraft, the answer must be "no".  Note however that Boulton appeared to envision using ailerons primarily to keep the wings level, while using the rudder as the primary turning control.  Note also that Boulton's patent application suggested that either the ailerons could either be manually operated, or a pendulum hanging below the aircraft could be used to pull on cords to activate the ailerons to automatically return the aircraft to wings-level whenever it entered a bank.  The latter concept would not have worked very well to keep the wings level, but might have had the unintended side effect of making the aircraft roll nicely into a bank whenever the pilot applied the rudder to try to turn, because the rudder would skid the aircraft and throw the pendulum off to one side!

"2) Regarding the adverse yaw, the description given in the patent and the explanation of Toulmin are quite  clear. The Wrights proposed a method for counteracting this unwanted effect but were they the first who noticed it?"

Yes, as far as I know no one is on record as having noticed adverse yaw before the Wrights.  Note that even weight-shift roll control can create adverse yaw, but Lilienthal's weight shift roll inputs probably involved too little cg displacement, and created too low a roll rate, to make enough adverse yaw to be easily noticed.

Matthew Piers Watt Boulton was aware of the notion of three-axis control as long as in his 1868 (see: https://commons.wikimedia.org/wiki/Category:Matthew_Piers_Watt_Boulton) patent he wrote about "a controlling power to direct the horizontal and vertical course of aerial vessels and to prevent their turning over by rotating on the longitudinal axis." (I quoted his words exactly.)

Thanks, Simplex11 -- someday I'll edit this answer to a) include an actual link (such as you provided) re Boulton, and b) credit you for bringing this to our attention...


text:
How much freight can a Boeing 737-800 NG carry if the majority of seats are removed?
In the context of the COVID-19 crisis, I read reports of underused Boeing 737-800 NG aircraft being converted (by technicians in airports, not by factories) to cargo planes for medical equipment. How much freight (in tons) can such an airplane transport? How much weight can the place below each seat withstand? The flight distance would be about 5000 km. It appears that one such airliner carried only 9 tons also, as can be seen in the images, it was loaded close to its maximum capacity. I guess the freight (200000 masks FFP2) was light.

Boeing 737-800 NG
VIDEO with such a Boeing 737-800 NG full of medical equipment

Press release:

9 (nine) tons of medical equipment brought from China by a Tarom aircraft - Bucharest, April 3, 2020. In the middle of the crisis generated by the coronavirus pandemic, a Boeing 737-800 NG aircraft has been reconfigured to import from China 9 tons of medical materials. The plane took off from Bucharest Otopeni airport on April 2 and through Almaty (Kazakhstan) reached Shenzhen (China) and then returned on the evening of April 3. Two Boeing 737-800 NG passenger planes have been reconfigured.

At 160 seats, a -800 carries 16 tons of people in the cabin, plus freight/luggage in the belly, plus fuel. At 180 seats, that becomes 18 tons of pax. If you assume one 50# suitcase per person, add a quarter of that again, so 20 to 22.5 tons.  Much more than that you're probably starting to run up against the Max Zero Fuel Weight limit, although I don't have the right charts in front of me to give an exact answer there.
If the flight is long enough, max takeoff weight becomes an issue, as your fuel load starts to cut into your lift. Obviously lots of factors go into how much fuel a flight requires; "today's" winds & temperature & destination weather (alternate required? How far away?) will change how much weight today's flight can take.
Ballparking it with back-of-the-envelope math, anything up to about 3-4 hours, you can probably get those weights I mentioned above (i.e. 22 tons or so). Much more than that, I'd expect max takeoff weight to limit you, so more distance = less payload.
If the legs were 5000km between refueling stops, then you're well into the limit being max takeoff weight, rather than what the jet could carry on a short-haul.
Depending on the nature of the cargo, the limits on floor loading in the cabin + the cargo compartments may be limiting, or volume may be limiting. If you're hauling pingpong balls, it's all about volume. If hauling gold, floor weight capacity & total weight will be more limiting. If it's all rucksacks, then you can probably leave the seats in & load around them, and only total weight will play.
Disclaimer, doing this with no references in front of me, and I may have a number off in there. When there is a specific need to haul "this" from A to B, the guys with the charts & load plans come up with a far more specific answer. The above is simply a rough ballpark.

The distance between Shenzhen and Almaty is 4112 km and between Almaty and Bucharest 4020 km. I guess the 737-800 NG needed fuel for at least 4500 km. This is the reason I said about 5000 km.

Fair enough... both of those legs are pretty long, so max takeoff weight will limit the weight they can load, much more than what they could take on shorter legs. Without some flight planning software, I don't have exact numbers. Suffice it to say, the weight you can load into the aircraft per se isn't the most limiting factor.

A Boeing 737-800 NG with all seats removed can not carry more than 24 000 kg of payload and the maximum volume transported can not surpass 184 m3, no matter what you do, as long as, according to the article B737-800BDSF Passenger to Cargo Conversion, "the 737-800BDSF [a factory modified 737-800 NG] is the second B737 New Generation converted freighter offered by IAI's Aviation Group" and it has the following characteristics:
Weights and Volumes:
MTW - Up to 174,700 lb
MTOW - Up to 174,200 lb
MZFW - Up to 138,300 lb
MLW - Up to 146,300 lb
Payload - 53,000 lb = 24000 kg
Fuel Capacity - 6,875 USG
Main Deck Cargo Volume - 4,999 ft3 = 141 m3
Lower Decks Bulk Volume - 1,543 ft3 = 43 m3

Source of images

What sort of operating weight (i.e. empty, with crew, zero fuel) does that thing have? If you subtract that payload from the max landing weight, you don't have much left for your landing fuel (as planned - before anything unexpected happens). Or does the conversion remove that much weight for seats, galleys, etc?

174,200 lb (MTOF) - 138,300 lb (MZFW) = 16,284 kg = 5,377 USG (Max fuel with Max abs payload = 24,000 kg).


text:
How do I determine the probable path of a future flight?
As a passenger on a regularly scheduled flight, how can I determine what areas my flight is likely to overfly?
I understand that inclement weather, other aircraft, and other outside influences (both somewhat predictable and completely unpredictable) can and will modify the planned route to some extent, which would be bonus information, but not necessary.

As mentioned in the linked question, FlightAware is a good start. It shows the actual routes of previous flights, and even overlays the radar image from that time to show if weather affected the routing.

In commercial flights, time and fuel are major concerns.  Therefore, flights go as direct as possible.  A straight-line path from origin to destination will be pretty close to the actual route flown.


text:
How does a PESA radar work?
I have been trying to understand how a PESA radar works. Essentially, it uses one signal that is electronically delayed. How is the reception coordinated and known by the receiver?
I am interested in understanding the logic behind in delay of signal and reception/identification.

My question tries to understand how different phases are obtained and how the reception of the coming signal is achieved

Phase control is obtained by adjusting the delay (think adjusting the length of the connection) between the source and the antenna. After the signal has been sent (an interference pattern), it doesn't matter if it came from a PESA, and AESA or a moving antenna.

How is the reception perform? I guess that the panel is static but surrounds the complete 360º with emissors? What are the emiters?

Yes, to cover 360° x 90° there will be 3 or 4 arrays, the maximum field of a flat array is an hemisphere. The signal is transmitted and received by the same individual antennas of the array (they can be any type of antenna, but likely a doublet or a patch.

Your question: What is the logic behind the delay of the signal and how is the echo signal later bound with the signal sent?

Short answer
A PESA is an array of individual antennas. Each antenna radiates the same signal, except the phase. The phase adjustment for each antenna allows for the non-moving array to behave like if it was rotated, i.e. the direction the energy is sent to or receive from can be selected by adjusting the phase for the individual antennas. In the example below, a simulation for a 5-element array, elements are separated by a distance of $\lambda / 4$ ($\lambda$ is the wavelength). The same phase offset is applied from one element to the next. By varying the offset, the direction of maximum strength is changed:

Varying the signal phase for an individual antenna of the array can be done by delaying the signal for this antenna, which in turn is as easy as making the electrical conductor (or waves guide) between the oscillator and the antenna longer.
PESA determines the transit time exactly like another more traditional one with a single rotating antenna. A signal sent by a PESA, after it has been sent, is indistinguishable from a signal sent by a rotating antenna radar (an array antenna can still be easily identified by its multiple side lobes, they are visible on the previous simulation).

Details
The principle of a primary radar is to send a radio signal in a given direction and wait for a possible echo. The direction of the radiation pattern is used in determining the direction (azimuth and elevation) of the reflecting object. The antenna must be very directional (small solid angle of the radiation pattern), else the angle determination cannot be precise.

Radiation pattern of an antenna with a parabolic reflector (source)
That means the antenna must be rotated to scan a useful portion of the sky (or ground).
To clarify the wording used, an antenna is usually composed of:

A radiator which receives the signal to be transmitted (it also collects the waves in reception). The radiator is the interface between electrons and waves and vice-versa.
A reflector used to reflect the waves from the radiator (it also focuses the waves on the radiator in reception).
Sometimes directors to narrow the radiation pattern of the system.

The radiation pattern of an antenna is mostly determined by the shape of the reflector (e.g. the parabolic reflector) which only illuminates a given fraction of the space and, due to interferences, also varies the energy based on the direction the energy is sent to or received from. Due to the reflector, a signal from the radiator will usually travels using different paths created by the multiple reflections.
Interferences are the result of combining the electromagnetic signals at a given point. For signals having the same frequency, the phase determines if signals are added (same phase) or canceled (opposite phases).

Phase determines how signals interfere (source)
We can apply this phenomenon to the multiple signals created in an antenna composed of a radiator and a cylindrical reflector.

Signals summation on a cylindrical reflector antenna creates a privileged direction of radiation with some secondary (unwanted) lobes (source)
Array of radiators (electronically scanned array, ESA)
Another way of varying the privileged direction of a radiation pattern is to create interferences on a voluntary basis, so that they create a stronger signal in the desired direction, and a weaker signal elsewhere. The advantage is that this electronic direction change can be much quicker than a mechanical rotation.
One way to create interferences is to send the radar signal from an array of small radiators instead of a single big one.

Close-up of the front of the old Cobra Judy radar (source)
This type of array is quite familiar under the name interferometer in the field of astrophysics:

Siberian Solar Radio Telescope (SSRT) interferometer observing the Sun at 5.7 GHz (source)
If all radiators are fed with the same signal, then interferences will occur the same way on the whole surface and the result will be that the direction perpendicular to the array is privileged (image on the left side).

Electronic beam-deflection (left: bore sight, right: deflected (source)
The right side of the image shows what happens if the signal sent to the individual radiators is delayed in a coordinated way: Delay increases from right to left on the array. It must be remembered that delay means phase here. The direction of the signal is now oblique due to the the location of constructive and destructive interferences. The red circles indicates signal with the same phase.
See this animation too.
Delaying an electric signal is quite easy: Just increase the length of the conductor (see analog delay line). A length variation between 0 and the wave length (e.g. 10 cm for a wavelength of 10 cm -- 3 GHz) allows to sweep the whole range of possible phase values from 0 to 360°.
Indeed there are other ways to delay a signal without adjusting the length, but they are electrically equivalent. When signals are digitized, it's really easy to process them, there is less electro-mechanical constraints.
By varying the phase on individual radiators we can steer the signal as if the antenna had been rotated, but we can also focus the energy on a target.

(source)
PESA vs AESA
There are two approaches for feeding individual radiators with phase-shifted signals: active and passive:

Active means the radiator has its own circuit to amplify its own signal. The active element must receive the value of the phase shift to use from a central element which coordinates the whole array.

(source)

Passive means the radiator receives the signal to be sent and just radiates it. The signal must already have the required phase. This is usually done with a single amplifier which feeds an array of phase shifters which in turn feed the array of radiators.

Transmission vs reception
The beam formed by the array concentrate the energy during transmission. The signal sent is not different than one sent with a single large radiator (rotating radar antenna). This is also a general result of adding and canceling energy by interference.
An array conforms to the general rule: An antenna is reversible, the characteristics of the radiation pattern are also valid in reception. That means:

Receive the same signal on all radiators
Add individual delays (phase shifts) to obtain a privileged direction of energy origin. The phase values are inverted compared to the ones used for sending in this direction.
Mix all signals into a single one to have them interfer and obtain a single mixed signal, equivalent to the one that would have been received with a rotating antenna.

The effect is that energy originating from directions other than the privileged one will be weakened, and the energy in the privileged direction will be emphasized.
Note that the phase is used to change the radiation pattern (process known as beam forming), but the rest of the radar is unchanged compared to a rotating single antenna one.
(Tell me if there is something that is not clear, I'll improve the post as necessary.)

Additional details
Phase shifter
As mentioned a phase shifter is used on each individual antenna of the array. It should prevent signal loss, distortion and noise addition, and the phase must be controlled accurately and quickly, by steps or continuously.
For step by step (digital), delay lines are used, with a possibility to connect a variable number of them using switching diodes (e.g. pin diodes). See more on RadarTutorial.eu with an example of 4-bit shifter (16 phase-shift values). More details on this datasheet for a 6-bit shifter on S-band.
Continuous (analog) control can rely on a resonant circuit (LC) to delay the signal. The capacity value can be mechanically changed in MEMS.

(source)
This extends past what I know in the domain, but other technologies exist, and are detailed in Array Phase Shifters: Theory and Technology (Nasa, 2007), including:

Another style of phase shifter uses coupled microstriplines as dc
electrodes to polarize a thin [...] ferro-electric film [...]. These
planar phase shifters are fairly compact, low loss, easy to fabricate,
and can provide 360° of phase shift with bias voltages under 350 V.

Phase shifters design depends on the operating wavelength of the radar.
Radiators used in a phased array antenna
Again the operating frequency impacts the radiators design.
Low frequencies may use:

A pair of crossed dipoles to allow polarization changes during the pulse travel and the reflection on the target.
A Yagi antenna.

Higher frequencies use other designs:

Notch antenna (slot). Used on Russian N011M (also note the dipole array for lower frequency scan of this hybrid radar).
Patch antenna (microstrip).
Vivaldi antenna (tapered slot) for broadband spectrum.

More on RadarTutorial.eu.

That's great, thanks. I still have some doubts... I will try to look them in internet or open another question

Your answer is world class. Honestly, I am impressed, thanks. After investigating a little bit I have only 2 doubts, the first is how effectively the phase is done, you mentioned the length of the conductor is changed, but I guess it is not done that way. How is achieved? The second one, is which devices are used as radiators both in active and passive? If you think is a new question and I can ask it.

@TrebiaProject.: Thanks a lot! I've added new elements (see at the bottom), but you could get better explanations with a new question.

Active does NOT mean that each radiator has its own oscillator to create its own signal.  Active just means that each has it's own transmit (HPAs) and receive (LNAs) amplifiers at the element level.

@SteveSh: Technically behind each element there is a switchable transmit/receive module (TRM) which consists in amplifiers and phase control. You're right in the sense there is a central source of RF power --thanks for saying it,-- this signal is then phase-shifted by each TRM to form the beam.

To further illustrate @mins' excellent explanation, here are some animations and simulations.
Quick Raytheon marketing vid of the Air and Missile Defense Radar, a next-gen AESA for destroyers. It provides volume search, horizon search, periscope search, ballistic missile tracking, and missile guidance and target illumination, among other functions. Go to [0:30]:

Another Raytheon AMDR illustration. Go to [1:00]:

Third AMDR animation by Lockheed Martin:

The F-35's APG-81 AESA (an advanced derivative of the F-22's radar) is demoed here in a marketing vid by Northrop Grumman, the maker. Interesting bit at [1:00].

See here for a simple 15 element linear array.


text:
What does "storing an aircraft" technically mean?
I have read on Wikipedia about airline fleets and you sometimes see that they have aircraft which they declare that have been stored.
Technically what does this mean? How do you store an aircraft? Since the plane is out in the open air and susceptible to weather, wouldn't delicate parts and even the fuselage degrade quite quickly?
Are stored airplanes then reused or resold in the future?
Wouldn't it be more feasible just to sell them instead of storing them for a period of time?

Depending on how long you are storing the plane, there are different options.
Some planes may be used or sold in the future. They are just waiting for the market conditions to improve so that the owner can afford to operate it again, or someone else would be willing to buy it. Some planes are too old for anyone to want to buy them. In this case, they are most useful to the current owner, so they store them in case they will be useful in the future. Another possibility is spare parts. While the plane is stored, it can be slowly disassembled to provide parts for other aircraft still in service. Once all useful parts are removed, it will be chopped up into scrap metal.
Recently, some new aircraft are being stored until they can find more pilots. There is also a case where the United States Air Force decided to purchase some planes, but later decided they didn't really need them. It's cheaper to send them to storage than cancel the contract, so straight to the desert they go.
You can rent hangar space somewhere to store the plane. This is much harder to do for larger planes, and hangar space is expensive and limited. However, it obviously offers the best conditions for the plane, where it can be protected from the weather. This is more likely to be a short term solution unless the plane is very important (or the owner has deep pockets).
Another option is the "boneyard:"

Areas like the US southwest have very low humidity year round, which reduces the amount of corrosion on an airframe. Airplanes are designed to be outside most of the time anyway, so they will hold up pretty well. Windows may be covered to keep the internal temperatures down, and certain gaps may be covered as well. Small airplanes may be completely wrapped in plastic.
Some examples of this include the boneyard at Davis Monthan AFB in Tucson, AZ, where the US military parks its planes, and nearby Pinal Airpark, where many civilian aircraft are stored.

Here there are some nice considerations.
You can basically divide stored aircrafts in two main categories: short-term and long-term.
From the link, for long-term:

When discussing parked or stored aircraft, it is safe to assume one is referring to an aircraft that is no longer in operation and has been placed by the owner, airline or lessor into a manufacturer-approved storage regime to preserve maintenance and asset value.

Instead, for short-term:

However, some will also refer to stored aircraft as those that are between owners. [...] a transition [...] takes time and [...] As such, an aircraft will often be stored and out of revenue service for six to 12 weeks.

or

Additional or unusually lengthy checks can delay the aircraft from revenue service for many months, during which time they are sometimes referred to as parked or stored.

but in the end, to answer why they are not sold:

With the exception of these transitions, the majority of aircraft in storage are there because they are no longer economically viable to operate; it is cheaper to put the aircraft into a storage programme than to fly it.

You keep it safe so that you can scrap it.

indeed if you fly it there is more maintenance involved than not flying it, and buying an end of life aircraft is throwing away money

It was a very hard decision to decide which answer to accept. Yours was a very good answer too but I preferred @fooot as I found it more spontaneous. If I could have split the vote I would have done so!!!


text:
Why is this plane circling over and over so late at night

Link to the specific flight on FlightRadar24
Why is this plane circling over and over so late?
Really interesting, love tracking planes

Beech Super King Air G-HMGC was registered to ACH London Ltd in June 2022. Website is listed as 2excelengineering.com, which describes the company:

2Excel Engineering Ltd is an independent, UK-CAA, EASA, FAA and TCCA approved Maintenance and Repair Organisation (MRO) for Boeing, Airbus, Beechcraft and Piper aircraft. 2Excel Engineering’s breadth, scale and flexibility enables it to undertake programmed ‘nose-to-tail’ maintenance, modifications and unscheduled work.

There are, however, multiple companies associated with this holding company. Another is 2ExcelAviation which states that:

It transforms small and large aircraft utility by designing, manufacturing, installing, testing and certifying modifications to the aircraft to make them useful to customers for their specific missions...(it) has certified a number of innovative approaches to create aircraft level installations ... King Air reconnaissance platforms.

The flight was likely testing some modification to the King Air. They seem to have a wide range of interest, but they specifically call out the recon platform for this aircraft. Circling would allow testing of observation equipment or the aircraft control needed by such equipment. They would test a new product in their own aircraft before offering or releasing it to customers.

Interestingly at least some of the tests seem to be done at very low altitude, that's what the yellow trail color means in Flightradar24. On a flight yesterday it took off from Humberside at 9:15 pm local time, climbed up to 17,000 feet and flew out over the middle of the English Channel, then descended to 800 ft and spent an hour an a half circling around above the water between 800 and 875 ft, then moved up to around 950 feet and circled for another half an hour, then headed back to Humberside where it arrived at 1:20 am local.

Ah thank you , I was watching it for a while and was interested what was happening.

"SRD224" seems to be UK Coast Guard ID. https://www.flightradar24.com/data/flights/srd224
Possibly training, or looking for someone.


text:
Is it the engines or the fuselage that limits service ceiling?
Reading this article about the Northrop Grumman E8 it says the the aircraft will be able to fly higher after engine upgrades. Wouldn't the max service ceiling be limited by the fuselage design too?

Service ceiling is defined as the maximum altitude at which the aircraft can maintain a specified rate of climb (typically 500 feet/minute for jets).
The service ceiling is dependent on a number of design factors like,

Engine power
Engine efficiency at high altitudes
Wing design

The service ceiling is usually limited by a number of factors, and engine is important among them.

Thrust (or power) decreases with altitude (density) and at some point the aircraft reaches an altitude where even at the 'maximum rate of climb' speed, the engine will not be able to produce enough (excess) thrust to get the required rate of climb. This will be the service ceiling. Usually this is the limiting factor for GA aircraft.
In case of very powerful engines, the wings may stop producing enough lift, though this is rarely the case.
In case of transport aircraft, the service ceiling may sometimes be limited by the maximum altitude from which they can descend to 14,000 ft in less than a specified time (4 min). Commercial airliners are usually limited by this certification altitude.
In commercial airliners, the cabin pressure altitude is held constant (usually ~8000 ft ISA) and the differential pressure will cause stresses on the fuselage, but this is usually not the primary concern as fuselage design incorporates elements (for e.g. relief valves) to relieve pressure when this differential pressure goes beyond a limit.

In short, usually it is the engine, rather than the fuselage that limits the service ceiling.

The only 2 problems of the fuselage is pressure and temperature. Pressure is not that much a problem, because the pressure difference will never be more than 1 atm. (The pressure at 50k ft is not that much lower than at 30k ft, in absolute numbers.) The temperature shouldn't pose a problem either.
However, the motors, they need oxygen to operate, and the higher you go, the less oxygen you get.


text:
Why do airlines use their own procedures instead of manufacturers'?
Why is it that airline A and airline B can have different (though similar) procedures for exactly the same operation and exactly the same aircraft?
Why don't they all use the same procedure since it is the same aircraft?
Isn't this a safety hazard with every airline doing their own thing?

An example would be useful

Beyond the Flight Manual, which is a certification document, the manufacturer's operational procedures (as per the Flight Crew Operating Manual) are basically an operational foundation that allows some customization.
There are variations from airline to airline in operational needs and internal culture, so different regulatory agencies allow airlines under their control to customize their procedures to suit the airline's safety and operational programs, as long as they meet certification requirements and the Regulator's own standards.
The variations are often to impose additional restrictions the airline feels are necessary.
A couple of examples:

OEM approves unrestricted use of flight spoilers for descent flaps up per the AFM.
Airline A allows use of flight spoilers for descent with no restrictions other than certification limits per the OEM's procedures.
Airline B's management feels that use of flight spoilers scares passengers, and/or wastes fuel, so it may to try to limit flight spoiler use in its own operations manual by incorporating some sort of language discouraging their use unless absolutely necessary.

I recall that on the CRJ200, the minimum speed flaps up per the AFM was 180kt. Some operators however, thought that was too low, and they modified their own operations procedures to limit flaps up speed to not less than 200kt.
You also see it with things like checklists. There is an OEM checklist, but most airlines just use it as a baseline and customize it to suit their needs, usually with extra steps, or possibly rearranging steps for better efficiency. The local Regulator will judge whether it meets requirements for safety.

Why would spoilers scare passengers? (Didn’t the airline post a spoiler warning?)

The rumble, quite noticeable on the CRJs. Some regional operators discouraged their use as the FAs would report pax that were alarmed. Also, if you are using flight spoilers, unless it's to deal with a surprise that forces you to descend quickly, you've basically screwed up the descent planning.

@JohnK - in the case of airlines that's probably a less common reason. A couple of times I have been on flights where the speed brakes were used for about three minutes during descent. In both cases I asked the pilot about it on the way out, in both cases they said ATC had kept them higher than normal as they approached the airport.

That's the case of "forced to descend quickly" I mentioned. But in general, aside from alarming pax, use of speed brakes is seen as a failure of energy management. That being said, on the CRJs the gear dropping is WAY scarier than speed brakes, if you're sitting over it.  BANG!  KA_LUNK!!!  It made me jump when I was sitting back there LOL.

@JohnK I think you may have missed the joke: "spoiler warning"... :)

@FreeMan Oh crap.  Yeah. LOL :0   Having no shame, I'll leave the comments up anyway...